Surface emitting semiconductor laser device

ABSTRACT

A surface emitting semiconductor laser device including a substrate, a bottom DBR, and a mesa post having a layer structure, the layer structure including a top DBR including a plurality of pairs, each of said pairs including an Al-containing high-reflectivity layer and an Al-containing low-reflectivity layer, an active layer structure sandwiched between the DBRs for emitting laser, and a current confinement layer disposed within or in a vicinity of one of the DBRs, the current confinement layer including a central current injection area and an annular current blocking area encircling the central current injection area, the annular current blocking area being formed by selective oxidation of Al in an Al x Ga 1−x As layer (0.95≦X&lt;1) having a thickness below 60 nm, the Al-containing low-reflectivity layer including Al at an atomic ratio not more than 0.8 and below 0.9. The progress of the oxidation in the Al-containing compound semiconductor layers can be suppressed during the formation of the current confinement oxide area by restricting the Al content in the specified range, thereby realizing the surface emitting semiconductor laser device having the longer lifetime, or the higher reliability.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a surface emitting semiconductorlaser device with a current confinement oxide area, and more inparticular to the surface emitting semiconductor laser device with thecurrent confinement oxide area having a longer lifetime, or a higherreliability.

[0003] (b) Description of the Related Art

[0004] A surface emitting semiconductor laser device emitting light in adirection perpendicular to a substrate attracts public attention in thedata communication field because of the possible arrangement of aplurality of the laser devices in a two-dimensional array on a singlesubstrate, different from a conventional Fabry-Perot semiconductor laserdevice.

[0005] The surface emitting semiconductor laser device includes a pairof DBRs (Distributed Bragg Reflector) (for example, Al(Ga)As/Ga(Al)As inthe GaAs-based reflector) and an active layer acting as an emittingregion sandwiched between the reflectors overlying a GaAs or InPsubstrate.

[0006] In order to increase the current injection efficiency and toreduce the threshold current, a surface emitting semiconductor laserdevice has been proposed having the current confinement structure formedby an Al oxide area.

[0007] For example, a GaInNAs-based material can be formed on the GaAssubstrate, thereby providing a AlGaAs-based DBR mirror having anexcellent thermal conductivity and a higher reflectivity. Accordingly,the mirror is promising in the surface emitting semiconductor laserdevice which emits light in a longer wavelength region from 1.2 to 1.6μm.

[0008] As shown in FIG. 1, a conventional 850 nm-range surface emittingsemiconductor laser device 10 includes a layer structure, overlying ann-GaAs substrate 12, having a bottom distributed Bragg reflector(hereinafter referred to as “DBR”) mirror 14 having 35 pairs ofn-Al_(0.9)GaAs/n-Al_(0.2)GaAs layers each having a thickness of λ/4n(“λ” is a lasing wavelength and “n” is a refractive index), a bottomcladding layer 16, a quantum well active layer 18, a top cladding layer20 and a top DBR mirror 22 having 25 pairs ofp-Al_(0.9)GaAs/p-Al_(0.2)GaAs each having a thickness of λ/4n.

[0009] In the top DBR mirror 22, one of the layers close to the activelayer 18 is formed as an AlAs layer 24 in place of the Al_(0.9)GaAslayer, and Al of the AlAs layer 24 in the area other than a currentinjection area is selectively oxidized to form a current confinementarea formed by an Al oxide area 25 which surrounds the current injectionarea.

[0010] The top DBR mirror 22 in the layer structure is configured to bea circular mesa post 23 having a diameter of 30 μm from the top to thelayer near to the active layer 18 formed by the photolithographic andetching process.

[0011] The annular current confinement area made of the Al oxide area 25is formed in the mesa post 23 by selectively oxidizing the Al in theAlAs layer 24 inwardly from the outer periphery of the mesa post 23 bymeans of the oxidation treatment of the layer structure at about 400° C.in a water vapor ambient. When, for example, the Al oxide area 25includes an annular ring having a width of 10 μm, the surface area ofthe central AlAs area 24 or the surface area for the current injection(aperture) is about 80 μm² having a circular shape with a diameter of 10μm.

[0012] The mesa, post 23 is surrounded by, for example, a polyimidesection 26, and a ring-shaped electrode acting as a p-side electrode 28is mounted in contact with the periphery of the top surface of the mesapost 23 by the width from 5 μm to 10 μm. After the thickness of then-GaAs substrate 12 is adjusted to about 200 μm by polishing the bottomsurface thereof, an n-side electrode 30 is formed thereon.

[0013] An electrode pad 32 for connection with an external terminal ismounted on the polyimide section 26 and in contact with the ring-shapedelectrode 28.

[0014] In the conventional current confinement oxide layer structure,the conversion of the AlAs layer into the Al oxide area by the oxidationcontracts the volume thereof to generate stress in the compoundsemiconductor layers adjacent to the Al oxide area. Thereby, the activelayer is deteriorated to reduce the lifetime of the device because theactive layer exists in the vicinity of the Al oxide area.

[0015] Thus, in order to prevent reduction of the device lifetime, useof an Al_(0.98)Ga_(0.02)As layer containing a smaller amount of gallium(Ga) has been proposed in place of the AlAs layer. Further, theprevention of the stress is attempted by decreasing the thickness of theAlAs layer to about 40 nm.

[0016] However, the oxidation rate is reduced by one order compared withthat of the AlAs layer having an ordinary thickness of 60 nm when theAl_(0.98)Ga_(0.02)As layer or the thinner AlAs layer is used.

[0017] Accordingly, in order to obtain the Al oxide area having the samewidth in the Al_(0.98)Ga_(0.02)As layer or the thinner AlAs layer, thetime of the oxidation should be increased or the oxidation temperatureshould be elevated.

[0018] As a result, as shown in FIG. 2, a problem arises that theAl_(0.9)Ga_(0.1)As layer having the higher Al content or the lowerrefractive index layer in the DBR mirror is oxidized in the annularshape along the periphery of the mesa post because the top DBR mirror isexposed to the severe oxidation conditions during the oxidation of theAlAs layer (current confinement layer).

[0019] The width of the oxide area formed in the DBR mirror depends onthe oxidation conditions including the composition and the thickness ofthe Al oxide area for the current confinement and the composition andthe thickness of the compound semiconductor layers in the DBR mirror,and at least about 2 to 5 μm is inevitably oxidized.

[0020] Although the oxidation amount of about 2 to 5 μm is smaller withrespect to the diameter of the mesa post, the volume contraction islarge enough to be neglected because the number of theAl_(0.9)Ga_(0.1)As layers constituting the DBR mirror is large and eachof the layers has an increased film thickness even if the oxide width isonly several μm. The stress generated due to the volume contraction mayadversely influence the reliability of the surface emittingsemiconductor laser device.

[0021] Since the thickness of the DBR mirror is λ/4n, the influenceincreases with the increase of the wavelength of the surface emittingsemiconductor laser device.

[0022] Further, as shown in FIG. 3, when a force is applied to the mesapost in the direction of the arrow shown therein, the stress isgenerated on the boundary between the AlAs layer 24 and the Al oxidearea 25 or the vicinity of the front edge of the Al oxide area 25 (nearto the center of the mesa post, a region “C” shown in FIG. 3).

[0023] Actually, the specimen of the surface emitting semiconductorlaser device which was compulsorily deteriorated through the reliabilitytest was observed with a transmission electro-microscope to confirm theoccurrence of rearrangement around the region “C” in FIG. 3.

[0024] The factors exerting the stress on the surface emittingsemiconductor laser device includes the external factor and the internalfactor. The external factors includes, for example, a dielectric, aprotection film made of polyimide and an electrode. Further, in themounting step of the surface emitting semiconductor laser device, thevarious forces may be applied to the mesa post.

[0025] The internal factor includes a distortion generated in the layerfilm after the crystal growth. Especially, the thickness of the layerstructure having the top and bottom DBR mirrors amounts to 10 μm in thesurface emitting semiconductor laser device, and the distortion in thelayer structure including the compound semiconductor layers is notnegligible.

[0026] Further, since the thickness of the compound semiconductor layersis established to be ¼n (wherein “n” is a refractive index) times thewavelength, the thickness of the DBR mirror is increased in the surfaceemitting semiconductor laser device having a longer wavelength range of1.2 to 1.6 μm. The thickness of the layer structure increases with theincrease of the DBR mirror to increase the distortion, therebysignificantly exerting the adverse influence to the lifetime of thesurface emitting semiconductor laser device.

[0027] A countermeasure for improving the device reliability includesthe reduction of the stress applied to the front edge of the Al oxidearea.

SUMMARY OF THE INVENTION

[0028] It is an object of the present invention to provide a surfaceemitting semiconductor laser device having a reliability obtained byreducing distortion or stress.

[0029] In a first aspect of the present invention (hereinafter referredto as “first invention), a surface emitting semiconductor laser deviceis provided which includes a substrate, a bottom DBR overlying thesubstrate, and a mesa post overlying the bottom DBR and having a layerstructure, the layer structure including a top DBR including a pluralityof pairs, each of said pairs including an Al-containinghigh-reflectivity layer and an Al-containing low-reflectivity layer, anactive layer structure sandwiched between the DBRs for emitting laser,and a current confinement layer disposed within or in a vicinity of oneof the DBRs, the current confinement layer including a central currentinjection area and an annular current blocking area encircling thecentral current injection area, the annular current blocking, area beingformed by selective oxidation of Al in an Al_(x)Ga_(1−x)As layer(0.95≦x<1) having a thickness below 60 nm, the Al-containinglow-reflectivity layer including Al at an atomic ratio not more than 0.8and below 0.9.

[0030] In accordance with the first invention, the progress of theoxidation in the Al-containing compound semiconductor layers can besuppressed during the formation of the current confinement oxide area byrestricting the Al content in the specified range, thereby realizing thesurface emitting semiconductor laser device having the longer lifetime,or the higher reliability.

[0031] In a second aspect of the present invention (hereinafter referredto as “second invention), a surface emitting semiconductor laser deviceis provided which includes a substrate, a bottom DBR overlying thesubstrate, and a mesa post overlying the bottom DBR and having a layerstructure, the layer structure including a top DBR including a pluralityof pairs, each of the pairs including an Al-containing high-reflectivitylayer and an Al-containing low-reflectivity layer, an active layerstructure sandwiched between the DBRs for emitting laser, and a currentconfinement layer disposed within or in a vicinity of one of the DBRs,the current confinement layer including a central current injection areaand an Al-oxidized annular current blocking area encircling the centralcurrent injection area, wherein the following relation holds:

a/b>0.6; and

b≧2 μm

[0032] where the “a” is a distance between the current confinement layerand top of the mesa post, and the “b” is a width of the Al-oxidizedannular current blocking area.

[0033] In accordance with the second invention, the height from the Aloxide layer to the highest layer of the mesa post (“a”) and the width ofthe Al oxide layer (“b”) are established to satisfy the specifiedrelation, thereby reducing the stress concentrated on the front edge ofthe Al oxide area to realize the surface emitting semiconductor laserdevice having the improved reliability.

[0034] The above and other objects, features and advantages of thepresent invention will be more apparent from the following description.

BRIEF DESCRIPTION OF DRAWINGS

[0035]FIG. 1 is a sectional view showing a conventional surface emittingsemiconductor laser device.

[0036]FIG. 2 is a schematic sectional view showing the progress of theoxidation of compound semiconductor layers forming a conventional DBRmirror.

[0037]FIG. 3 is a schematic view showing the stress concentrated on thefront edge of an Al oxide area when a force is applied to a mesa post.

[0038]FIG. 4 a schematic view showing the relation between the heightfrom the Al oxide layer to the highest layer of the mesa post (“a”) andthe width of the Al oxide layer (“b”).

[0039]FIG. 5 is a sectional view showing a surface emittingsemiconductor laser device in accordance with Embodiments of the presentinvention.

[0040]FIG. 6 is a graph indicating the relation between a devicelifetime and the oxide width of a DBR mirror in Example 1.

[0041]FIG. 7 is a graph indicating the relation between an oxidationrate and an Al content “X” in Example 2.

[0042]FIG. 8 is a graph indicating the relation between the devicelifetime and the width “b” in Example 3.

PREFERRED EMBODIMENTS OF THE INVENTION

[0043] In the first invention, the Al content of the compoundsemiconductor layer constituting the multi-layer film reflector isbetween 0.8 and 0.85 inclusive, and the side oxidation of themulti-layer film reflector is below 2.5 μm from the outer periphery ofthe mesa post. The diameter of the mesa post is preferably between 30and 40 μm inclusive.

[0044] The oxide width having such a value does not exert the adverseeffect on the lifetime of the surface emitting semiconductor laserdevice.

[0045] The first invention can be applied to any surface emittingsemiconductor laser device having the current confinement oxide layerand the multi-layer film reflector formed by the compound semiconductorlayers containing the Al, especially to that having the multi-layer filmreflector of Al_(X)Ga_(1−X)As/Al_(X)Ga_(1−X)As overlying the GaAssubstrate regardless of the material of the active layer and theemitting wavelength.

[0046] In the preferred embodiment of the first invention, the activelayer structure having the emitting wavelength range of 850 nm or 1200to 1600 nm overlies the GaAs substrate.

[0047] In connection with the second invention, the present inventorshave investigated the relation between the force and the width of the Aloxide area, and the force is exerted on the boundary between the Aloxide area and the original AlAs layer or exerted on the front edge ofthe Al oxide area.

[0048] The relation will be described referring to FIG. 4 in which theheight from the Al oxide layer to the highest layer of the mesa post(“a”) and the width of the Al oxide layer (“b”) are indicated.

[0049] As shown in FIG. 4, the force applied to the point “C” increaseswith the increase of the width “b” in accordance with the principle oflever if the force applied on the outer periphery of the mesa post. Whenthe “a” is larger, the force applied to the point “C” increases if theforce applied on the highest layer of the mesa post.

[0050] In the above-described 850 nm-range surface emittingsemiconductor laser device (corresponding to 25 pairs ofAl_(0.9)Ga_(0.1)s/p-Al_(0.2)Ga_(0.8)As), the “a” is preferably about 3.2μm and the “b” is preferably about 10 μm.

[0051] In the meantime, the reduction of the “b” which can decrease theforce applied to the front edge “C” of the Al oxide area wrongfullydecreases the optical confinement effect to lower the threshold currentand the current injection efficiency.

[0052] However, in accordance with the present inventors' investigationby using the specimen of the edge surface emitting semiconductor laserdevice, the optical or current confinement function was not deterioratedwhen the width of the Al oxide area was 2 μm or more as described inN.Iwai et. al, IEEE J. Select. Topics Quantum Electron., 5,(1999), 694.

[0053] Based on the above, the optimum range of the width “b” is 2 and 5μm inclusive when the width “a” is 3.2 μm (850 nm range). However, thepermitted range of the “b” increases with the increase of the “a”.

[0054] The present inventors have found that, after the repeatedexperiments, the relation of a/b>0.6 is required to improve thereliability.

[0055] For example, in the surface emitting semiconductor laser devicewith the wavelength of 1300 nm having the width “a” of about 5 μm, thewidth “b” may be 8.33 μm or less. Further, in the surface emittingsemiconductor laser device with the wavelength of 1550 nm having thewidth “a” of about 6 μm, the width “b” may be 10 μm or less.

[0056] The configuration of a surface emitting semiconductor laserdevice in accordance with embodiments of the present invention will bedescribed referring to the annexed drawings.

[0057] Embodiment 1

[0058] As shown in FIG. 5, a surface emitting semiconductor laser device40 of Embodiment 1 includes substantially the same configuration as thatof the conventional surface emitting semiconductor laser device 10 ofFIG. 1 except that an Al-containing layer to be oxidized for forming anAl oxide area constituting a current confinement structure is anAl_(0.98)Ga_(0.02)As layer (central current injection area) 42 having athickness of 60 nm in place of the AlAs layer 24 of FIG. 1, an Al oxidearea (annular current blocking area) 44 is formed by selectivelyoxidizing Al in the Al_(0.98)Ga_(0.02)As layer, and a multi-layer filmreflector constituting a DBR mirror 46 includes 25 pair ofp-Al_(0.85)Ga_(0.15)As/p-Al_(0.2)Ga_(0.8)As in place of the 25 pair ofp-Al_(0.9)GaAs/p-Al_(0.2)GaAs of FIG. 1.

[0059] In Embodiment 1, the stress between the Al oxide area 44 and theadjacent p-Al_(0.2)Ga_(0.8)As layer 42 is reduced by using thep-Al_(0.85)Ga_(0.15)As layer as the Al-containing layer to be oxidized,thereby increasing the device lifetime. Further, the oxide width of thep-Al_(0.85)Ga_(0.15)As layer constituting the DBR mirror 46 is made tobe 2.5 μm or less, thereby further increasing the device lifetime andthe reliability of the device.

[0060] An Al_(X)Ga_(1−X)As (0.95≦X≦1)layer having a thickness below 60nm can be used as the Al-containing layer to be oxidized.

[0061] Embodiment 2

[0062] As shown in FIG. 5, a surface emitting semiconductor laser device40 of Embodiment 2 includes substantially the same configuration as thatof the conventional surface emitting semiconductor laser device 10 ofFIG. 1 except that an Al oxide area 42 is used having a width between 2and 5 μm inclusive, for example 5 μm in, in place of the Al oxide area25 having the width of 10 μm in FIG. 1, and the diameter of a mesa postis 20 μm for making the diameter of a central AlAs layer 44 or thediameter of a current injection region (aperture) to be 10 μm.

[0063] In Embodiment 2, the height “a” from the Al oxide area 42 to thehighest layer of the mesa post 23 is 3.2 μm.

[0064] When the surface emitting semiconductor laser device has anemitting wavelength range of 1300 nm, the width of the Al oxide layer(“b”) is 8.3 μm≧b≧2 μm because the height from the Al oxide layer to thehighest layer of the mesa post (“a”) is about 5 μm.

[0065] When the surface emitting semiconductor laser device has anemitting wavelength range of 1550 nm, the “b” is 10 μm≧b≧2 μm becausethe “a” is about 6 μm.

EXAMPLE 1

[0066] Surface emitting semiconductor laser devices having DBR mirrorswith various oxide widths were fabricated by using, as a compound oxidelayer for forming an Al oxide area for the current confinement, anAl_(0.9)GaAs layer or an AlAs layer having a thinner thickness below 60nm followed by the conversion thereof into the Al oxide areas undervarious oxidation conditions. The relation between the oxide width (ofthe layer having a higher Al content) of the DBR mirror and the lifetimeof the surface emitting semiconductor laser device was investigated by alife test thereof conducted at 85° C. The device lifetime was defined tobe a period of time from initial to a point at which the optical outputwas reduced by 2 dB under the operation with the specified drivingcurrent injection.

[0067] The results shown in a graph of FIG. 6 indicate that the devicelifetime could be increased by reducing the oxide width of the DBRmirror. Further, the practically required device lifetime, or thereliability of the surface emitting semiconductor laser device, could beobtained by suppressing the oxide width of the DBR mirror to be 2.5 μmor less.

EXAMPLE 2

[0068] The present inventors have investigated a method of reducing theoxide width of the DBR mirror.

[0069] The oxidation rate of the Al_(X)Ga_(1−X)As layers forming the DBRmirror depends on the Al content “X”, and increases with the increase ofthe Al content “X”. Accordingly, the reduction of the oxide width can beachieved by reducing the Al content “X” and increasing the oxidationselectivity of the AlAs layer. However, the reduction of the Al content“X” decreases the difference of the refractivities between theAl_(X)Ga_(1−X)As layer and the other Al_(X)Ga_(1−X)As layer. Formaintaining the specified reflectivity, the number of the pairs of theDBR mirrors should be increased, thereby elevating the fabrication cost.

[0070] In this manner, the Al content “X” of the DBR mirror and thenumber of the pairs of the DBR mirrors or the fabrication cost are inthe inverse proportion.

[0071] In Example 2, the relation of the relative oxidation rate wasinvestigated between the Al_(0.98)GaAs layer or the AlAs layer having athinner thickness below 60 nm acting as the compound semiconductor layerfor forming the Al oxide area for the current confinement and the Alcontent “X” constituting the DBR mirror. The results are shown in agraph of FIG. 7 in which the abscissa indicates the Al content “X” inthe AlGaAs layer and the ordinate indicates the relative oxidation ratewith respect to the standardized oxidation rate of AlAs of 1.0 (Alcontent is 1.0), thereby showing the relation between the Al content “X”and the oxidation rate.

[0072] It is apparent from FIG. 7 that the oxidation rate is nearlyconstant in the Al content “X” not more than 0.85, or it can beconcluded that the optimum value of the Al content “X” in the AlGaAslayer is in the range having the higher Al content “X” and having thehigher selectivity of the oxidation rate, or the range from 0.8 to 0.85.Accordingly, the maximum value of the Al content “X” in which theoxidation rate is not increased is 0.85.

[0073] While the graph of FIG. 7 shows the relation between the Alcontent “X” and the oxidation rate of the AlGaAs layer, the relation isalso applicable to the oxidation of the compound semiconductor layercontaining Al.

EXAMPLE 3

[0074] The reliability test of the surface emitting semiconductor laserdevice was conducted by intentionally exerting the force on the point“C” under the conditions that a temperature was 100° C. and theinjection current was 200 mA in the ACC driving

[0075] The device lifetime was defined to be a period of time frominitial to a point at which the optical output was reduced by 2 dB.

[0076] The results are shown in a graph of FIG. 8 in which the ordinateindicates the device lifetime of the test and the abscissa indicates thewidth of the oxide layer. The device lifetime was nearly constant whenthe “b” was 8 to 10 μm. However, the device reliability was largelyimproved when the width “b” was 5 μm.

[0077] This is because, as shown in FIG. 3, the reduction of is thewidth “b” decreases the force applied to the front edge “C” of the Aloxide area. The device reliability is elevated when the width “b” ismade to be 5 μm or less.

[0078] Since the above embodiment is described only for examples, thepresent invention is not limited to the above embodiment and variousmodifications or alterations can be easily made therefrom bay thoseskilled in the art without departing from the scope of the presentinvention.

What is claimed is:
 1. A surface emitting semiconductor laser devicecomprising a substrate, a bottom DBR overlying the substrate, and a mesapost overlying the bottom DBR and having a layer structure, the layerstructure including a top DBR including a plurality of pairs, each ofsaid pairs including an Al-containing high-reflectivity layer and anAl-containing low-reflectivity layer, an active layer structuresandwiched between the DBRs for emitting laser, and a currentconfinement layer disposed within or in a vicinity of one of the DBRs,the current confinement layer including a central current injection areaand an annular current blocking area encircling the central currentinjection area, the annular current blocking area being formed byselective oxidation of Al in an Al_(x)Ga_(1−x)As layer (0.95≦x<1) havinga thickness below 60 nm, the Al-containing low-reflectivity layerincluding Al at an atomic ratio not more than 0.8 and below 0.9.
 2. Thesurface emitting semiconductor laser device as defined in claim 1,wherein the mesa post has a diameter between 30 and 40 μm, and theannular current blocking area has a width of 2.5 μm or smaller.
 3. Thesurface emitting semiconductor laser device as defined in claim 1,wherein the substrate is a GaAs substrate, and the active layerstructure has an emission wavelength of 850 nm range.
 4. The surfaceemitting semiconductor laser device as defined in claim 1, wherein thesubstrate is a GaAs substrate, and the active layer structure has anemission wavelength between 1200 and 1600 nm.
 5. A surface emittingsemiconductor laser device comprising a substrate, a bottom DBRoverlying the substrate, and a mesa post overlying the bottom DBR andhaving a layer structure, the layer structure including a top DBRincluding a plurality of pairs, each of the pairs including anAl-containing high-reflectivity layer and an Al-containinglow-reflectivity layer, an active layer structure sandwiched between theDBRs for emitting laser, and a current confinement layer disposed withinor in a vicinity of one of the DBRs, the current confinement layerincluding a central current injection area and an Al-oxidized annularcurrent blocking area encircling the central current injection area,wherein the following relation holds: a/b>0.6; and b≧2μm where the “a”is a distance between the current confinement layer and top of the mesapost, and the “b” is a width of the Al-oxidized annular current blockingarea.
 6. The surface emitting semiconductor laser device as defined inclaim 5, wherein the active layer structure has an emission wavelengthof 850 nm range, and the “b” satisfies 5 μm≧b≧2 μm.
 7. The surfaceemitting semiconductor laser device as defined in claim 5, wherein theactive layer structure has an emission wavelength of 1300 nm range, andthe “b” satisfies 8.3 μm≧b≧=2 μm.
 8. The surface emitting semiconductorlaser device as defined in claim 5, wherein the active layer structurehas an emission wavelength of 1550 nm range, and the “b” satisfies 10μm≧b≧2 μm.